In electrohydraulic automatic transmissions, it is generally customary to monitor the methodical operation in order to prevent situations critical to safety. The input signals made available by the sensors, the electromagnetic actuators, the electronic transmission control and, by fed back values, the clutches and brakes that participate in a gearshift, are monitored. In connection with this, EP-PS 0 316 358 discloses a safety system in which the automatic transmission switches to the highest gear in case of an error. The electromagnetic actuators and hydraulics are disposed in a manner such that even in the event of a total electronic system failure, the automatic transmission switches to the fifth gear.
A safety system for an automatic transmission is also known from DE-PS 27 00 548. It prevents a downshift from the highest gear by disengaging an electromagnetic valve when a fault occurs.
The prior art described is disadvantageous in that momentary faults immediately result in a switch to the highest gear or in maintaining the highest gear. In practice this means that, for example, a switch from the first to the fifth gear occurs in the extreme case.
The problem to be solved by the invention is thus to provide a safety system for an automatic transmission, which detects temporary faults and reacts with adequate flexibility.